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1. Field of the Invention
The present invention relates generally to methods and apparatus for achieving and maintaining acceptable indoor air quality.
2. Description of Related Art
One industry that is mature and economically sensitive to costs is the heating, ventilation and air conditioning (HVAC) industry. Because of the competitive nature of both the construction and HVAC industries, HVAC systems must be inexpensive to purchase and install. Of a more global interest though, is the cost to operate and maintain HVAC systems. Often, a building owner will replace an aging HVAC system as the reduction in operating and maintenance costs can offset the retrofit cost, sometimes in a matter of months.
HVAC systems are typically comprised of a cooling and heating section for, respectively, cooling and heating the air. An HVAC system will also include fans and ductwork for moving this conditioned air where it is needed. In most HVAC systems, air is drawn in, filtered, cooled and dehumidified or heated and humidified, and then delivered to a space. The greatest portion of this air is drawn from the space for recirculation through the HVAC system.
One factor impacting design and operation of HVAC systems is indoor air quality (IAQ). A major consideration in IAQ today is the amount of outdoor air introduced by an HVAC system into an otherwise sealed space. The HVAC industry and others have adopted standards for the introduction of outdoor air into spaces serviced by an otherwise closed HVAC system. These include offices, residential, commercial, industrial and institutional spaces, as modes of transportation such as cars, buses, planes and ships. In addition to controlling indoor air for occupant comfort, the goal of most HVAC systems is to provide air with reduced levels of particulate, gases and bioaerosols, be it for semiconductor, pharmaceutical or food processing facilities, hospitals, schools or offices and now the home.
Various reasons have contributed to the lack of success in utilizing germicidal lamps for bioaerosol control (IAQ), except for limited and specialized purposes. The functional implementation of such devices in air moving systems has been limited generally to expensive portable units with questionable efficacy. However, non-moving air devices can be found as wall or ceiling mount systems where the germicidal lamp is situated in a minimum air movement, and proper ambient air temperature area. A typical germicidal tube is designed to operate in still air of 80-90xc2x0 F. to maintain a tube wall temperature of 105xc2x0 F. Germicidal lamps have sensitive physical characteristics including plasma gas(es), mercury and partial pressures thereof.
When a conventional germicidal lamp is used to irradiate moving airstreams, the air moving across the tube removes heat and lowers the tube""s temperature. The tube""s mercury begins to condense such that the emission of the germicidal wavelength of 253.7 nm decreases. This decrease can be up to 75% when the tube wall temperature reaches 58xc2x0 F. Also, at lower internal temperatures, tube components degrade quicker, shortening tube life. This phenomenon, known as skin effect cooling, requires a notable increase in the number of conventional tubes required for a given level of performance. Increasing the number of tubes reduces the available square area for airflow. This in turn requires the airs"" velocity to increase, which decreases the dose (time times intensity) and air volume. If such a system could be made to work, it would require an increase in fan horsepower, UVC light energy and in the number of expensive tube replacements.
Conventional germicidal lamps emit ultraviolet light at both the primary and secondary emission lines of mercury (254 nm and 187 nm). At mercury""s 187-nm line, ozone is created and in many applications of germicidal lamps, such as in water, this is desirable. However, ozone has strict threshold limit values in air due to its strong oxidative properties and harm to humans. Also, numerous companies have attempted to apply germicidal lamps to HVAC systems, these conventional germicidal lamps have proved unsatisfactory. Typically, a conventional germicidal lamp performs best when installed in a device or room where the air is still and/or warm. So despite the clear benefits of germicidal lamps, problems such as decreased output in moving and/or low temperature air, reduced air changes and ozone production have prevented their use in all but specialized environments.
Germicidal fixtures continue to enter the HVAC market. Recent entries have been sold under the Germ-O-Ray and Germitroll trademarks. The particular capabilities and design of these devices is not known to the inventors, though it is believed both devices use conventional tubes so that when installed in air ducts, they will suffer from the criteria outlined above.
For further information concerning improvements in electric discharge devices, which are directed to overcoming such problems, reference is made to the above-identified patent applications. These other patent applications describe excellent devices and methods for using germicidal lamps to make HVAC systems more efficient, less costly to operate and maintain, and to provide better IAQ for a healthier environment.
Germicidal tubes differ significantly from electric discharge devices used in ultraviolet gas spectroscopy (VUV tubes). Germicidal tubes are low-pressure types that emit UV light at the primary and secondary emission lines of mercuryxe2x80x94254 nm and 187 nm. In contrast, VUV tubes are high-pressure types that operate at high temperatures and as a consequence, emit different spectral lines and intensities.
In occupant air one group of gas phase contaminants are classified as volatile organic compounds (VOCs). VOCs have been associated with simple unpleasant odors to serious maladies. Many people can detect even low part per billion (ppb) concentrations of VOCs in the air, and VOCs can be found in concentrations of parts per million (ppm). Numerous studies show the human nose to be the best gas chromatograph and further that many people have mild to serious sensitivities to certain or mixed VOCs and their associated odors. In higher concentrations, some VOCs can cause physical discomfort and maladies requiring medical attention. Since newer buildings have become more energy efficient (tighter), internally generated VOCs are of greater concern.
Some level of VOCs and other organic odors have existed in new and old buildings alike for decades. Mechanically ventilated spaces accumulate simple organic gas phase compounds as a result of operating office equipment, adding new building materials or furnishings and using various cleaning agents and solvents to name a few.
When attempting to rectify IAQ problems, gaseous contaminants can be diluted (controlled) through the introduction of outside air. However, diluting VOCs with outdoor air is neither efficient nor cost effective. It requires both more heating and cooling to condition this air and it may bring in more pollutants than it dilutes.
Other prior art methods include filtering air through activated carbon or activated alumina encapsulated by potassium permanganate, to either adsorb the VOCs or to chemically react with them in an effort to break them down (oxidize). Both of these methods have certain disadvantages. Both filtering devices require additional space and structure within the ventilation system as they can be 24xe2x80x3xc3x9724xe2x80x3xc3x9724xe2x80x3 for every 2000 cfm and weigh over 110 pounds each. Additionally, they require added system static pressure (in air horsepower) to move air through them. Both require lots of natural resources to either reactivate or dispose of as hazardous waste fill. The initial cost to install these filters, excluding labor, is approximately $850 for every 2000 cfm. Their maintenance costs are from $290 to $400 annually. A properly designed activated carbon system lasts approximately 12 months. A properly designed potassium permanganate encapsulated activated alumina system lasts approximately 9 months. Thus, at least once per year, these special filters require expensive, hazardous and intrusive service. These systems also require more air horsepower to move air through them and thus more energy consumed. When they are added to an existing system, it could necessitate speeding up the fan and/or changing out the fan and fan motor to a larger size.
Outside of HVAC, VOC-control has been pursued through several techniques. One technique uses liquids to wash VOCs from a gas stream. However, these liquid systems are inadequate for treating air in an HVAC system. They can be more costly and hazardous than the filter systems described above. Heat treatment using radiant beds or afterburners has been used to partially catalyze VOCs in certain applications. However, heat treatment is not compatible with HVAC systems. The amount of heat that is added would also have to be offset by added cooling capacity. Photocatalysis has been gaining popularity in high VOC concentration atmospheres but again first and operating costs are prohibitive. Solvent recovery systems utilizing high volumes of activated carbon are the extreme and here we are simply dealing with a misapplication.
UVC at predominately 253.7 nm in and of itself has not been considered for VOC control. One prior art method used ultraviolet light of 185 nm to produce ozone for breaking down odor. In that prior art method, an air stream was passed across an ultraviolet lamp of UV energy at 185 nm where oxygen (O2) is separated forming unstable O1""s, which combine with O2""s to form O3""s or ozone. Also, it was found that when an ultraviolet lamp was placed in a moving airstream, the reduced UV output was insufficient enough to reduce the production of both 185 nm generated ozone and 253.7 nm UVC to have much effect on most VOCs. Thus, reliance only upon ultraviolet light even when producing ozone was considered wholly inadequate for VOC-control.
The previously described problems are solved in a method and apparatus for reducing ppb concentrations of volatile organic compounds and common organic odors to below threshold limit values in a mechanically ventilated space. The mechanically ventilated space has a mechanical ventilation system comprising plural ducts and an air moving apparatus such as a fan. Ultraviolet radiation is introduced into a duct to treat an air stream passing by the UV radiation and moving through the duct. The air stream passes at a speed of at least 100 to over 1500 feet per minute and has a temperature of between 30xc2x0 and 90xc2x0 F. An untreated air stream could have a concentration of volatile organic compounds as high as 100 parts per million.
A low-pressure germicidal lamp is installed with respect to the interior of the duct such that the germicidal lamp, when energized, will irradiate the air stream. The germicidal lamp when energized emits ultraviolet radiation of approximately 254 nm with a power of at least 30 to 3000 microwatts/cm2 at 1 meter for every 4 square feet of duct area, with substantially no ozone generated. The UV ionization radiation separates many volatile organic compounds at the molecular and atomic level to water vapor and carbon dioxide thereby lowering the concentration of volatile organic compounds to 90 parts per million and down to 10 parts per billion.
Still further objects and advantages attaching to the device and to its use and operation will be apparent to those skilled in the art from the following particular description.